Field of Art
The disclosure relates to an optical system and an image pickup apparatus including the same. The disclosure is applicable to an image pickup apparatus employing an image pickup element, such as a digital still camera, a video camera, a monitoring camera, and a broadcasting camera, or an image pickup apparatus, such as a camera using a silver-halide film.
Description of the Related Art
A telephoto image pickup optical system in which an optical system having positive refractive power is disposed on an object side, and an optical system having negative refractive power is disposed on an image side is known as an image pickup optical system with a long focal length. The telephoto image pickup optical system is used in a super-telephoto lens of fixed focal length, for example.
In a super-telephoto lens, many lenses are generally disposed on an object side to desirably correct various aberrations. Since the lenses disposed on the object side have large effective diameters, a weight of the entire optical system tends to increase due to an increase in number of lenses disposed on the object side.
Generally, as the focal length increases, an axial chromatic aberration and a magnification chromatic aberration become more notable. Especially, fluctuation in the chromatic aberration upon focusing is notable. Japanese Patent Laid-Open No. 2015-215561 discloses an optical system in which a positive lens and a negative lens are disposed in a focus unit which has negative refractive power and which moves upon focusing, in which a material of high anomalous dispersion is employed as a material for the positive lens, and a material of low dispersion is employed as a material for the negative lens.
In the optical system described in Japanese Patent Laid-Open No. 2015-215561, the focus unit is disposed at a position relatively close to a lens disposed closest to an object. Therefore, effective diameters of lenses included in the focus unit tend to be large. In the optical system described in Japanese Patent Laid-Open No. 2015-215561, a negative lens is disposed in the focus unit to desirably correct a spherical aberration occurring in a positive lens included in the focus unit.
Therefore, in the optical system described in Japanese Patent Laid-Open No. 2015-215561, since the focus unit is constituted by a plurality of lenses having large effective diameters, the focus unit tends to increase in size and the optical system tends to increase in weight.